The present invention claims priority from Japanese Patent Application No.9-306011 filed Nov. 7, 1997, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a micro wave integrated circuit (MIC) having a plurality of circuit parts mounted on a ceramic substrate and a method for manufacturing the same integrated circuit. The present invention is utilized in a communication equipment, radar, measuring equipment or other equipment. Particularly, the present invention is utilized in a filter circuit, an isolator, a transmission/receiving separating circuit, an antenna circuit or other circuits of this type of equipment. Although the present invention was developed for utilization thereof in a high frequency circuit operating at a frequency exceeding 10 GHz, the utilizing frequency is not limited thereto and the present invention can be utilized in a circuit operating at a frequency higher than 10 GHz.
2. Description of Related Art
A technique for forming a micro wave integrated circuit having a plurality of circuit parts mounted on a ceramic substrate by soldering or ribbon or wire connection has been known. Although it is usual to form conductors of a circulator, among the circuit parts mounted on a ceramic substrate, on a soft ferrite substrate, Japanese Patent Application Laid-open No. Hei 61-288486 discloses a technique in which holes are formed in a green sheet of an alumina substrate which is to be sintered at low temperature, ferrite parts are fitted in these holes and the green sheet is sintered to fixedly secure the ferrite parts in the holes.
This technique is a useful technique in mounting ferrite parts in a ceramic substrate and the present invention is based on this technique. However, this technique is to form a circulator part by mounting a ferrite unit on a low temperature sintered alumina substrate and does not suggest a technique for arranging a plurality of mutually related circuit parts on a low temperature sintered alumina substrate to unify them as an integrated circuit. Further, the disclosed technique does not suggest a use of hard ferrite as the ferrite material and a magnetization of the hard ferrite correspondingly to circuit characteristics after sintering.
On the other hand, Japanese Patent Application Laid-open No. Hei 6-334413 of the same assignee as that of the present invention discloses a technique for unifying a high frequency filter as a monolithic micro wave integrated circuit. According to this technique, an insulating film is formed on a surface of a semiconductor substrate and conductor lines are formed on the insulating film. Further, a dielectric resonator is mounted in the vicinity of the conductor lines. The dielectric resonator is fitted in a recess formed in the insulating film.
However, according to this technique, it is impossible to unify a conventional circulator using soft ferrite.
On the other hand, a high frequency band exceeding 10 GHz has been used in various equipment. Particularly, a frequency of an automobile radar or a distance meter is assigned to a 60 GHz band or a 70 GHz band. Therefore, in view of limited space in which such a device is mounted, a technique for mass-producing a compact high frequency integrated circuit having uniform characteristics at low cost is required. In the conventional MIC technology, however, an area for connecting such parts as a circulator, high frequency filter and MMIC, etc., is necessary, with which there is a limitation in reducing the size of the device.
In reducing the size of the device, the conventional technique is basically superior in forming a high frequency filter integrally with the MMIC. However, the integration of a conventional circulator with the MMIC is impossible in principle since the conventional circulator requires an external magnet. Further, in order to obtain uniform characteristics according to the conventional MIC technology, the parts themselves must have stable characteristics. Even if such parts are obtained, loss which can not be considered in the design stage of the device may occur in connecting portions of the parts and it is impossible to control such loss, so that there may occur variation of characteristics between substrates. Therefore, it is necessary to regulate the characteristics of the device after it is assembled, the number of working steps is increased and skilled workers are necessary for the regulation of the characteristics of the device.
The present invention was made in view of the above background and has an object to provide a micro wave integrated circuit (MIC) including dielectric parts and magnetic parts both formed on a single substrate and a method for manufacturing the same.
Another object of the present invention is to provide a reliable high frequency integrated circuit having substantially no power loss at connecting points which is the problem in the structure in which individual parts are connected and a method for manufacturing the same high frequency integrated circuit.
Another object of the present invention is to provide a high frequency integrated circuit which has stable characteristics and is capable of being manufactured uniformly and a method for manufacturing the same high frequency integrated circuit.
Another object of the present invention is to provide a high frequency integrated circuit capable of being manufactured with a small number of manufacturing steps and a method of manufacturing the same high frequency integrated circuit.
A further object of the present invention is to provide a high frequency integrated circuit which can be mass-produced at low cost and a method of manufacturing the same high frequency integrated circuit.
Another object of the present invention is to provide a high frequency integrated circuit which is suitable for use in designing and manufacturing a filter or an antenna circuit which can be utilized in several tens GHz frequency band and a method for manufacturing the same high frequency integrated circuit.
A further object of the present invention is to provide a high frequency integrated circuit whose electric characteristics can be reversibly regulated while monitoring the characteristics thereof after manufacture and a method for manufacturing the same high frequency integrated circuit.
According to a first aspect of the present invention, a high frequency integrated circuit is featured having a structure in which magnetic parts or magnetic parts and dielectric parts are unified in a single integrated circuit substrate and in which two or more of these circuit parts form desired high frequency characteristics. That is, a plurality of parts constructing an isolator, a circulator or a filter are constituted as a circuit having high frequency characteristics as a whole. A conductor pattern is formed on a surface of the ceramic substrate as lead lines of the circuit or as micro strip lines.
The magnetic parts may be of hard ferrite. The hard ferrite can be utilized as a constructive element of a circulator or an isolator. Further, the dielectric parts can be utilized as constructive elements of a high frequency filter. The dielectric substrate may be of organic material and, particularly, polytetrafluoroethylene is a suitable material thereof. The dielectric substrate is preferably a ceramic substrate.
According to a second aspect of the present invention, a method for manufacturing a high frequency integrated circuit, which is featured by comprising, in a case where the dielectric substrate is a glass ceramic substrate, the steps of forming a plurality of holes in a green sheet of the glass ceramic, fitting circuit parts in the respective holes, forming the ceramic substrate by sintering the green sheet at a temperature not higher than a temperature at which the circuit parts are deformed and not lower than a sintering temperature of the green sheet and fixedly securing the circuit parts in the ceramic substrate by utilizing the shrinking nature of the holes during the sintering process of the ceramic substrate. The circuit pats include magnetic parts and dielectric parts and these parts are unified in a single substrate. The sintering temperature of the ceramic substrate is preferably in a range from 800xc2x0 C. to 1200xc2x0 C.
The height of the circuit part is not smaller than a thickness of the ceramic substrate and the hole is circular. An outer configuration of at least a portion of the circuit part is preferably a circular rod.
When some of the magnetic parts are of hard ferrite, the sintering temperature of the green sheet is not higher than the sintering temperature of the hard ferrite and the magnetic parts of hard ferrite are magnetized by applying magnetic field thereto after the sintering of the ceramic substrate.
The Curie temperature of hard ferrite is within a range from 400xc2x0 C. to 700xc2x0 C. depending upon the nature thereof and the magnetizing step is preferably performed during a cooling period for cooling the substrate from about Curie temperature to room temperature after the sintering of the ceramic substrate is completed. It is, of course, possible to perform the magnetization or demagnetization of the magnetic parts or to regulate the magnetized state after the ceramic substrate is cooled to room temperature. In the magnetizing step, the electric characteristics of the integrated circuit is preferably regulated while monitoring it by regulating magnetic field applied to the magnetic parts of hard ferrite.
A plurality of holes are formed in the glass ceramic substrate in a green sheet state before sintering by punching and the circuit parts are fitted in the respective holes. Then, the substrate is sintered. By forming the holes by punching, the holes can be in a uniform and precise mutual positional relation. Since the green sheet is shrunken by sintering, the mutual positional relation of the holes is regulated by testing a plurality of samples such that the final configuration of the green sheet after sintering becomes a desired one. Once the mutual positional relation of the holes is regulated, it is possible to produce a large number of integrated circuits having identical configuration without necessity of regulating it individually.
The size of hole formed in the green sheet is also regulated by testing a plurality of samples such that the magnetic parts and the dielectric parts are rigidly secured to the holes after sintering. Once the size of hole is regulated, it is possible to produce a large number of integrated circuits uniformly.
The height of the circuit part (length of the parts measured in a vertical direction from the ceramic substrate surface) is preferably equal to or larger than the thickness of the substrate. Particularly, the height of the magnetic part is preferably the same as the thickness of the substrate, since it is usual to form a conductor between the magnetic part and the substrate and the characteristics of the integrated circuit may be degraded if there is a step in the conductor. When the height of the dielectric part is larger than the thickness of the substrate, it is also possible to machine it. In such case, the freedom of configuration of the part becomes large and it becomes possible to design various circuits.
It is most preferable to make the hole provided in the green sheet circular and at least a portion of the circuit part which is to be fitted in the hole circular rod since the tightening force of the hole with respect to the rod portion of the circuit part becomes uniform. The outer configuration of the circuit part may be other than circular rod, such as ellipsoidal. In such case, the number of tests to be performed in order to make the firm tightening optimal may be increased.
By using hard ferrite as the material of the magnetic parts, the external magnet becomes unnecessary. When an isolator or a circulator is realized by applying a magnetic field to a magnetic element thereof by the external magnet in a high frequency integrated circuit operating in a frequency band of several tens GHz, the magnetic field may influence other elements close to the aimed magnetic element since the integrated circuit is small and the inter-element distance is small, so that the mutual relation becomes complex, causing the design thereof to be difficult. However, by using the hard ferrite, the influence of magnetic field on other elements is minimized and it becomes possible to realize a desired characteristics by a relatively simple design.
The sintering temperature of the green sheet should be selected such that it changes the natures of the magnetic parts, particularly, the hard ferrite. According to experiments conducted by the inventors, it has been found that, even in a case where the sintering is performed at a temperature equal to or higher than Curie temperature of hard ferrite, desired characteristics can be obtained by lowering the temperature after the sintering is completed and regulating the magnetic orientation again by applying magnetic field to the hard ferrite when the temperature of the hard ferrite becomes about the Curie temperature or lower than the Curie temperature. Since the sintering temperature and the Curie temperature of hard ferrite is not uniform depending upon the kind of ferrite, the temperature in the sintering step and the temperature in the magnetizing step are selected according to the nature of the hard ferrite. It is not always necessary to set the sintering temperature equal to or higher than the Curie temperature of hard ferrite.
When the magnetization is performed after the sintering is completed, it may be possible to magnetize after the sample temperature becomes room temperature. However, it is easily possible to magnetize the magnetic part to a desired value when the magnetization is performed while lowering the temperature from a relatively high temperature. It is further possible to change the magnetizing state while monitoring the characteristics of the high frequency circuit by operating the high frequency circuit after being magnetized once.